1. Field of the Invention
The present invention relates to a variable speed drive for driving a transmission mechanism of an internal combustion engine.
2. Description of Background Art
Conventionally, a variable speed drive for driving a transmission mechanism of an internal combustion engine includes a shift motor for rotating a shift spindle for the automatic transmission. See, for example, Japanese Patent Laid-Open No. 2011-208766.
In the structure disclosed in Japanese Patent Laid-Open No. 2011-208766 (hereinafter referred to as prior art), an inputting mechanism and shift position changing driving means, which is an operation mechanism, are disposed in such a manner that they are distributed to the left and right sides of a crankcase which serves also as a transmission case. The inputting mechanism is configured from a shift motor and a speed reduction mechanism for receiving a rotational driving power of the shift motor. Meanwhile, the operation mechanism includes a master arm which pivots integrally with rotation of the shift spindle and a stopper member for limiting the amount of the pivotal motion of the master arm by being abutted with the master arm. The shift spindle is connected to the inputting mechanism and the operation mechanism and extends leftwardly and rightwardly through the inside of the crankcase. Thus, the rotational driving power of the inputting mechanism is transmitted to the operation mechanism through the shift spindle.
In the prior art, when the shifting speed is to be raised, the master arm is pivoted at a higher pivoting speed. Thereupon, the master arm is abutted with and stopped by the stopper member. However, if the pivoting speed of the master arm is high, then the impact when the master arm is abutted with the stopper member is high, and this high impact is transmitted to the inputting mechanism through the shift spindle. In order to suppress transmission of the impact, in the prior art, the axial length of the shift spindle is made long so that the shift spindle can be twisted readily thereby to suppress transmission of the impact to the inputting mechanism. Further, as the twist angle of the shift spindle between the inputting mechanism side and the operation mechanism side increases, a greater difference appears between rotational angles of the shift spindle between the inputting mechanism side and the operation mechanism side. In order to accurately detect the angle of rotation of the axial end of the shift spindle on the operation mechanism side, a rotational angle sensor is attached to the axial end of the shift spindle on the operation mechanism side. By adopting such a configuration as just described, the prior art can achieve a shifting operation of a high accuracy at a high speed by an automatic transmission.
In addition, depending upon the layout of an internal structure of an internal combustion engine, the inputting mechanism and the operation mechanism for the shift spindle may have to be disposed on the same side on the outer side of the transmission case.
In this case, the axial length of the shift spindle becomes short and the shift spindle becomes less likely to be twisted. Therefore, if the pivoting speed of the master arm is high, then when the master arm and the stopper member are abutted with each other, the impact applied to the master arm is less likely to be absorbed sufficiently by the shift spindle. Thus, there is the possibility that the impact may be transmitted to the inputting mechanism through the shift spindle. If the impact is transmitted to the inputting mechanism, then there is the possibility that an influence may be hand on the accuracy of the inputting mechanism. In order to raise the strength of the inputting mechanism to avoid the influence, it is necessary to increase the thickness or the diameter of gears, which gives rise to a problem in upsizing and so forth of the inputting mechanism.
Further, if the rotational angle sensor is attached to the axial end of the shift spindle on the operation mechanism side, then the rotational angle sensor is disposed in the inside of the transmission case. Therefore, it may be difficult to keep the temperature of the rotational angle sensor lower than an upper temperature limit. Consequently, it is sometimes desired to attach the rotational angle sensor to the inputting mechanism side. However, where the rotational angle sensor is attached to the inputting mechanism side, if the shift spindle has a twist angle, then the accuracy of the rotational angle sensor may degrade (the value of the sensor which detects the rotational angle of the shift spindle on the inputting mechanism side may be displaced from the actual rotational angle of the shift spindle on the operation mechanism side). Thus, there is a problem that it is difficult to carry out a shifting operation at a high speed and with a high degree of accuracy.